Solid electrolytes have recently been used more often than the conventional electrolyte (aqueous) solutions. This is firstly because those solid electrolytes have good proccessability in application in electric and electronic materials, and secondly because of the transitions to overall size and weight reduction and electric power saving.
Inorganic and organic proton conductive materials are known in the art. The inorganic proton conductive materials, such as uranyl phosphate hydrate, show inadequate bonding properties in the interface to cause many problems in forming a conductive layer on a substrate or an electrode.
On the other hand, the organic proton conductive materials include polymers that belong to the so-called cation exchange resins, for example sulfonated vinyl polymers such as polystyrene sulfonate; perfluoroalkylsulfonic acid polymers; and perfluoroalkylcarboxylic acid polymers. In particular, proton conductive membranes based on the perfluoroalkylsulfonic acid polymers represented by Nafion® (manufactured by Du Pont Kabushiki Kaisya) have been widely used. The perfluoroalkylsulfonic acid polymers possess high proton conductivity and can improve the forming property that has been a drawback of the inorganic conductive materials. However, on the other hand they are very expensive and are still unsatisfactory in terms of usability and mechanical properties at high temperatures. Therefore, a material has been desired that is more inexpensive, has thermal and mechanical stability and shows excellent ion conductivity as solid electrolyte membranes.
Approaches to the aforesaid problems include use of organic polymers such as polymers occurring by introducing a sulfonic acid or phosphate group in heat resistant polymers such as polybenzimidazole and polyether ether ketone (see non-patent documents 1 to 3).
Meanwhile, the organic proton conductive materials proposed so far include sulfonic group-containing polyarylenes. Such polymers are generally produced by polymerizing an aromatic compound into a base polymer and introducing sulfonic acid in the base polymer.
In general, the polymer basic skeleton including units from an aromatic compound provides a solid electrolyte membrane that is more stable thermally and mechanically than those obtained from the perfluoroalkylsulfonic acid polymers. However, the solid electrolyte membranes made of such polymers are excessively inflexible because of the aromatic skeleton or the sulfonic acid introduced in the main chain, and therefore they have bad proccessability. Specifically, it is often the case that the films are more brittle and exhibit lower bonding properties with carbon electrodes in MEA (membrane-electrode assembly) fabrication than films of the perfluoroalkylsulfonic acid polymers.    [Non-patent document 1] Polymer Preprints, Japan, Vol. 42, No. 7, p. 2490-2492 (1993)    [Non-patent document 2] Polymer Preprints, Japan, Vol. 43, No. 3, p. 735-736 (1994)    [Non-patent document 3] Polymer Preprints, Japan, Vol. 42, No. 3, p. 730 (1993)